06-05-2010, 04:43 AM
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The pleco will get quite large (and produce a lot of waste!) so be prepared for that. If you keep the water quality high, he should be able to live fairly comfortably in a 75g tank. Is he swimming in that picture, or just sitting still? His fins are clamped, which can be a sign that there's something "wrong" if he's just sitting still. Upping the water quality through water changes might make him perk up.
But yes, the shark is not only brackish but eventually needs full marine conditions as an adult fish and is very predatory. Unfortunately, that completely rules it out as a friendly freshwater community fish.